The polymerization of olefins, particularly ethylene, is well known and has been a widely practiced commercial art for many decades. Catalysts for such polymerization are well known to include Zeigler type catalysts. In the Zeigler type catalyst field, the catalyst is usually made up of a transition metal compound and an alkyl aluminum, which is used as a co-catalyst, sometimes with a magnesium compound as well, usually on a suitable support.
It is conventional commercial practice to polymerize ethylene using a catalyst comprising the reaction product of a titanium halide, suitably titanium tetrachloride; an aluminum alkyl, suitably triethyl or trimethyl aluminum; and a magnesium containing compound, suitably a Grignard reagent, or a magnesium halide salt, all deposited on a suitable substrate particle carrier material, usually defined as a solid, porous material, such as silica, silica-alumina, and combinations thereof. In this field it is common for the catalyst/support particle to become part of the final polymer product and to leave the polymerization reaction zone as a composite with the polymer which has been produced.
In the ethylene polymerization art, it has been well known for a long time to copolymerize ethylene with various higher alpha olefins, such as propylene, butene, hexene and octene, in order to produce linear low density polyethylene (LLDPE). LLDPE resins possess properties which distinguish them from other ethylene polymer resins, such as ethylene homopolymers. Certain of these desirable properties are set forth in U.S. Pat. No. 4,076,698, issued to Anderson et al.
This copolymerization to produce LLDPE is widely commercially practiced. Different manufacturers use different polymerization systems, such as for example: solution, slurry or gas phase polymerization reactions. The gas phase copolymerization reaction is described in U.S. Pat. No. 4,302,566 to Karol et al.
The catalyst which is used in these co-polymerizations is often heterogeneous, and is made by depositing suitable catalytic materials onto the surface of a suitable porous substrate. Since most of the surface area of porous materials exists within the pore structure of these materials, deposition on the surface includes deposition on the internal surface, that is on the internal surface of the pores, as well. As a general proposition, titanium, and sometimes vanadium, are the transition metals of choice. Titanium is usually the commercial norm. In those situations where the catalyst is heterogeneous, the transition metal, suitably titanium, has usually been deposited on a silica substrate. U.S. Pat. No. 4,137,547 to Graff, describes a supported catalyst obtained by treating a support with both an organo-aluminum compound and an organo-magnesium compound followed by contacting this treated support with a tetravalent titanium compound.
As an alternative approach to making a suitable polymerization catalyst, U.S. Pat. No. 3,787,384 to Stevens et al. and U.S. Pat. No. 4,148,754 to Strobel et al. describe a catalyst prepared by first reacting a suitable support (e.g. silica containing reactive surface hydroxyl groups) with an organo-magnesium compound (e.g. a Grignard reagent) and then combining this reacted support with a tetravalent titanium compound.
It should be noted that the traditional wisdom in making catalysts for the copolymerization of ethylene and higher olefins is to use a tri- or a tetra-valent titanium compound, usually in combination with aluminum and/or magnesium. This catalyst is usually assembled in a heterogeneous manner by impregnation of the catalytic materials onto a particulate silica support. There are many references which disclose many different means of maximizing the utility of these catalysts by changes in formulation as well as by specifying the characteristics and the physical properties of the silica substrate, either as to particle size, pore size, pore size distribution, or surface hydroxyl concentration, etc.
In more recent times, it has been discovered that the copolymerization of ethylene and higher olefins, that is the production of LLDPE, can be catalyzed by special zirconium and/or hafnium compounds. These compounds, called metallocenes, have been proposed for use in this service in combination with aluminoxanes, as co-catalysts, both deposited on a silica substrate.
In this regard, reference is made to published European patent applications 294,942 and 313,386 both in the name of Mitsui Petrochemical Company, and to U.S. Pat. No. 4,808,561 to Wellborn, Jr. These references have made various proposals for the formulation of suitably improved catalysts, and many additional publications have been developed dealing with differently formulated and improved catalysts based on these metallocenes, particularly zirconocenes, and aluminoxanes. In this regard, reference is here made to the list of references cited in the above referred to U.S. Pat. No. '561 patent, all of which are incorporated herein by reference.
In most of the published art on heterogeneous catalysts for the polymerization of ethylene alone or in combination with other higher .alpha.-olefins, the catalyst has been suggested for use deposited on a substrate particle. In most publications, the substrate particle is reported to be an inorganic refractory oxide, suitably silica, alumina, or silica-alumina.
In some publications, mention has been made of using organic substrates in combination with titanium halide catalysts for olefin polymerization. Reference is here made to U.S.S.R. Author's Certificate numbered 682262 and an application for a Supplement thereto numbered 2,187,995/23-04. In the published version of this application to Supplement, it is said that polyethylene and polypropylene particles, which have substantially inert surfaces, and copolymers of ethylene with vinyl alcohol, which has active surface sites, have been previously known, in combination with titanium, to catalyze this polymerization.
Both of these substrate materials have been indicated by these references to be unsatisfactory for one or more reasons. According to this publication, however, it is noted that styrene homo-and copolymers, notably copolymers with divinyl benzene, can be used as a carrier for a vanadium olefin polymerization catalyst. This carrier material is said to be useful because the vanadium tetra chloride catalyst is reacted with the aromatic rings of the polystyrene, thereby being converted to vanadium trichloride, which is said to be an active catalyst.
The copolymers with divinyl benzene are said to be cross-linked and to have internal pore structures with physical properties which depend on the degree of cross-linking. The reaction of the vanadium tetra chloride with the polystyrene forms a solid phase of the resultant vanadium trichloride on the surface, including within the pores, of the polystyrene. This product is alleged to be combinable with metallorganic compounds, such as aluminum alkyls, to form a highly active catalyst for the stereospecific polymerization of ethylene and propylene.
On this same note, U.S.S.R. application 1,886,351/23-04 shows the use of a titanium trichloride/aluminum alkyl catalyst similarly reacted with and deposited on a polystyrene carrier for the polymerization of propylene. The catalyst is deposited from a titanium tetrachloride form in much the same way as described above for the vanadium tetrachloride.
Reference is here made to published European patent application 283,011, the disclosure of which corresponds to U.S. Pat. No. 4,900,706, assigned to Sumitomo Chemical Company. In this publication, there is disclosed an olefin polymerization catalyst comprising titanium, magnesium and chlorine, and possibly an organo-aluminum co-catalyst, all deposited on an organic polymer carrier. The thrust of this reference is that the carrier should be porous particles having a mean diameter of 5 to 1,000 .mu.m, a pore volume of at least 0.1 ml/g and an average pore radius of 100 to 5,000 .ANG.. It is to be noted that the preferred polymer substrate of this reference includes styrene-divinyl benzene co-polymer.
In U.S. Pat. No. 4,808,561, it is mentioned that the support could be organic in nature, such as a resinous support material like a polyolefin. There is generally disclosed in this reference the possibility of using finely divided polyethylene as a support material for a zirconocene-aluminoxane based ethylene polymerization catalyst.
However, in this U.S. Pat. No. '561 patent, the catalyst is more particularly described as having been made by reacting an aluminoxane and a metallocene in the presence of a solid refractory support member. Although, as noted above, in this patent there is a general disclosure that finely divided polyethylene could possibly be used as a suitable substrate for this new type of catalyst, all the specifics of the rest of the disclosure in this patent are directed to the use of silica as the catalyst substrate.
In U.S. Pat. No. 4,921,825, there is a disclosure of using particulate organic polymer supports for ethylene polymerization catalysts. In this patent there is a disclosure in the specific examples of the use of polyethylene powder and of spherical polystyrene powder as supports.
In the body of the specification of this U.S. Pat. No. '561 patent, it is said that the catalyst substrate should be a particulate oxide which should be surface dehydrated such that it is substantially free of absorbed moisture, and therefore be as inert as possible. Beyond this, it is said that the specific particle size, surface area, pore volume, and the number of surface hydroxyl groups are not critical to the utility of the silica as a substrate in the preparation of the zirconocene-aluminoxane catalyst which is suited to use in that invention.
In the past, one of the instant inventors has published, in U.S. Pat. No. 4,876,229, that, in co-polymerization of ethylene with higher .alpha.-olefins using catalysts comprising titanium on silica, there is an effect of pore size of the substrate on the effectiveness of the polymerization catalysis. In this regard, it is now generally accepted that, as the pore size of the silica substrate decreases, the molecular weight distribution of the copolymer products decreases. It is also generally accepted that, as the pore size of the silica substrate increases, titanium based catalysts tend to become more effective. That is, as the pore size of the silica substrate increases, the catalyst has higher activity, better co-monomer incorporation, and better hydrogen chain transfer utilization.
In marked contrast to this, where a cross-linked styrene-divinyl benzene particle substrate was used to support a titanium polymerization catalyst, for reasonable pore volumes of at least about 0.5 ml/g, no relationship could be determined between the pore size and pore size distribution of the substrate and the effectiveness of the catalyst for the co-polymerization of ethylene with hexene-1. This is consistent with the disclosures of the above cited Russian references which similarly did not note any such relationship. The content of the above cited Furtek patent, and particularly the examples thereof, fully support the conclusion that, while there may be a relationship between the pore size of a silica substrate and the effectiveness of a titanium catalyst deposited thereon, there does not appear to be any substantial relationship between the pore size of a resinous styrene-divinyl benzene cross-linked copolymer porous substrate and the effectiveness of the same titanium based catalyst deposited thereon.
It is well known that ethylene polymers made using zirconium metallocene-aluminoxane based catalysts are different from ethylene polymers made using tetravalent titanium halide catalysts, even where all of the other polymerization operating parameters are attempted to be held substantially constant. For many applications, the ethylene polymers, particularly the copolymers of ethylene with higher .alpha.-olefins, made with a zirconocene/aluminoxane catalyst are preferred.
The instant inventors have simultaneously herewith filed an application for patent directed to an improved catalyst system useful for the polymerization of ethylene, particularly for the co-polymerization of ethylene with higher .alpha.-olefins. It is an important aspect of that copending application that the resinous substrate of the catalyst thereof is a co-polymer of monomers which are well suited to being cross-linked during polymerization or after having been polymerized. The cross-linked, co-polymer substrate particles disclosed in the examples of this copending application have substantially no surface functionality. After polymerization and cross-linking, the surfaces thereof are, indeed, inert.